Non-invasive techniques for measuring various blood parameters, e.g. blood oxygen saturation, have become very popular, since they do not require the withdrawal of a blood sample from a patient's body. Optical monitoring techniques of the kind specified utilize the detection of light transmitted or reflected from the location on the patient's body under measurement, and are based on spectrophotometric measurements enabling the indication of the presence of various blood constituents based on known spectral behaviors of these constituents.
Most of these techniques utilize a measurement optical device or probe, designed in a manner to be attached to the patient's finger, which includes an optical assembly for irradiating the finger with light and detecting its light response. The conventional devices of the kind specified, such as a pulse oximeter, which is the generally accepted standard of everyday clinical practice, provide for measuring enhanced optical pulsatile signals caused by the changes in the volume of a blood flowing through a fleshy medium (e.g., finger).
It is known that for blood parameters other than oxygen saturation, e.g., glucose concentration, significant difficulties have been accoutered, because their absorption spectral behavior in red and near infrared regions is not as remarkable as for the oxygenized homoglobin. Hence, the main limitations on the way of expanding the non-invasive techniques to the measurements different from pulse oximetry are associated with the limited selectivity of the absorption based method.
A different technique is disclosed in U.S. Pat. No. 6,400,972, WO 01/45553 and WO 01/96872, all assigned to the assignee of the present application. This is an occlusion-based technique, according to which an over-systolic pressure is applied to the blood perfused fleshy medium with a normal blood flow so as to create a state of temporary blood flow cessation at the measurement location. The measurement with different wavelengths of incident radiation and/or different polarization states of detected light are carried out at timely separated sessions taken during a time period including a cessation time when the state of the blood flow cessation is maintained. This technique utilizes the condition of the “artificial blood kinetics” rather than the natural blood kinetics taking place when the state of the total blood cessation is not achieved. As a result of the cessation of the blood flow, a condition of the artificial kinetics is achieved with the optical characteristics of the blood associated with the light response being different from those at the natural blood kinetics. Indeed, it is known that the scattering properties of blood depend on the size and shape of scatterers (aggregates). Thus, time changes of the light response at the condition of artificial kinetics depend on the changes in the shape and average size of the scattering centers in the medium, i.e., red blood cells (RBC) aggregation (Rouleaux effect). It was found that owing to the effect of the artificial kinetics, the optical characteristics of blood changes dramatically, such that they differ from those of the fleshy medium with a normal blood flow by about 25 to 45%, and sometimes even by 60%. Hence, the accuracy (i.e., signal-to-noise ratio) of the technique based on the artificial kinetics as well as selectivity of the optical measurements can be substantially better when compared with those based on measurements of the blood parameters at natural kinetics.
Various probe devices suitable for the occlusion-based measurements are described in U.S. Pat. No. 6,213,952 and US 2002/0173709 both to the assignee of the present application. These devices are designed to apply over-systolic pressure to the patient's finger at a location upstream of a measurement location in the finger, with respect to the direction of normal blood flow, thereby creating a state of blood flow cessation at the measurement location. Generally, the device includes a clip member for securing the patient's finger between its two clamping arms that also serve for carrying the optics of a measurement unit, and includes a pressurizing assembly outside the clip member for applying over-systolic pressure.